1. Field of the Invention
The present invention relates to tungsten based heavy alloys, and more particularly to a method for making irregular tungsten/matrix interfaces in tungsten heavy alloys by cyclic heat treatment and resintering.
2. Discussion of the Background
Tungsten heavy alloys consist of greater than 90% by weight of tungsten, and nickel and iron. These alloys are usually manufactured by a liquid phase sintering, a powder metallurgic method, because of high melting temperature of tungsten. Tungsten heavy alloys have a good combination of high density and strength. Therefore, these alloys are widely used for rotors and weight balance, as well as for a penetrator of an armor piercing fin stabilized discarding sabot.
FIG. 1 shows a typical tungsten heavy alloy microstructure.
As shown therein, spherical hard tungsten grains (white portions) of a BCC structure are surrounded by soft matrix phase. So these alloys are one of the metal matrix composites(MMCs) comprising distinct the two phases and interfaces of tungsten/matrix and tungsten/tungsten, respectively. In recent years, it was reported that thermal stresses are induced in the tungsten heavy alloys 1! during heating and cooling, due to the mismatch in the thermal expansion coefficient(TEC) between the tungsten grain and matrix phase. When cooling, tensile and compressive stresses are introduced at the tungsten/matrix interfaces and tungsten/tungsten grain boundaries and vice versa. Therefore, reduction of tungsten/tungsten grain boundary area in heavy alloys can be obtained by a repetitive heating and quenching, which is called a cyclic heat treatment at a usual heat treatment temperature. The result of the cyclic heat treatment is the penetration of matrix between tungsten/tungsten grain boundary, and a drastic increase in impact energy 1!. By increasing the number of heat treatment cycles for fixed total heat treatment time, tungsten/tungsten grain boundary area decreased.
Meanwhile the interface problems in tungsten heavy alloys have long been studied, because the mechanical properties of the alloys are closely related with interface problems. These problems are caused by impurities and morphological change at tungsten / matrix interfaces.
The impurities of sulfur, phosphorus and carbon at the tungsten / tungsten and tungsten / matrix interfaces can be healed by heat treatment or addition of suitable scavengers, such as calcium and lanthanum.
A morphological change in the tungsten/matrix interface was observed in by adding a fourth element, such as Mo and Re. Addition of Mo or Re to the starting W, Ni and Fe powders often resulted in an irregular tungsten grain shape because the dissolution rate between W and Mo or Re into the matrix is different. On the other hand, it is also known that the shape change at the tungsten/matrix interfaces can occur by plastic deformation and post annealing process. In this case, however, recrystallization of tungsten grains and matrix penetration into the tungsten/tungsten interface are inevitable. So it has been known that the irregular tungsten/matrix interfaces cannot be obtained in tungsten heavy alloys without adding a 4th element or applying a plastic deformation.
In the present invention, we developed a new type of irregularity at the tungsten/matrix interface in the tungsten base heavy alloys. To accomplish this object, we evaluated the effect of a cyclic heat treatment, i.e. thermal stress accumulated by the repetitive heating and quenching, and post resintering, on the irregular shape change at the interfaces.